Thread structures used on containers can take a wide variety of designs. The details of any one particular thread structure on a container is influenced by many factors, including the contained contents, operational aspects of the complimentary closure, materials, methods of package manufacture and consumer use.
A particularly useful and widely accepted closure/seal system for packages is to position external threads on the container which mate with internal threads positioned on the interior wall of a closure. As is well known, the closure is removed and reapplied by rotary threading action.
One factor requiring attention with threaded closure systems is the circumferential extent of mating thread engagement between closure and container. One may desire to minimize circumferential thread engagement to only that required for adequate closure retention for a number of reasons. These include avoiding requirements for excessive turning during closure manipulation by the consumer. Moreover, equipment associated with rotary capping operations is normally restricted in the number of “turns” of the closure allowed during initial application. On the other hand, there must be enough thread engagement for proper threading and sealing on application. A common “rule-of-thumb” in classic packaging technology is that at least a single turn of thread engagement should be incorporated into the designed thread engagement between the fully applied closure and container. This “rule-of-thumb” is most often adequate for packaging using classic materials and fabrication, such as combinations of rigid glass containers and rigid polystyrene or polypropylene closures. In these cases the complimentary threads have been designed to be relatively massive (such as the familiar modified buttress design) with substantial thread depth. In this way the required surface contact between the topside of the closure thread and the underside of the container thread is normally achieved with one turn (360 degrees) of complimentary thread engagement.
It is common to deviate from the “classical” packaging designs, materials, and fabrication for a myriad of reasons, such as, to provide lightweight packaging by thinning the wall sections and structural improvements. However, when providing lightweight packaging other concerns such as part flexibility and distortion are increased. Another example is the choice of alternate materials such as low density polyethylene (LDPE) for the closure, taking advantage of the unique properties of LDPE. In these cases, if one wishes to employ a threaded closure, the classic one turn “rule-of-thumb” may not be adequate to ensure proper retention of the applied closure. This is a result of the added flexibility of thin walling or the inherent relative flexibility of the LDPE materials. In some cases a minimal amount of internal container pressure, such as that experienced when the container may be dropped, is sufficient to cause the closure skirt to expand to the point where the closure simply pops off. This flexibility can also allow localized distortion of the closure to the point where the closure threads “strip” relative to the mating container threads. This stripping action normally initiates at the bottom end of the closure thread where the hoop strength of the closure is at a minimum. At that position, radial distortion of the closure skirt allows disengagement of the mating threads. Continued torquing causes the disengagement to proceed helically upward in a “tiring” manner until finally the mating threads “jump” over each other. This stripping mechanism is not only of concern on initial application, where such stripping can result in an unseated closure, but also in the hands of the consumer expecting reseal integrity.
In order to adjust for the inherent flexibility of LDPE materials, designers have often chosen to dramatically increase the circumferential extent of mating thread engagement. However, when maintaining a single lead thread, the amount of turning required to apply and remove the closure can become excessive for rotary capping and/or convenient consumer manipulation. These concerns can be addressed by using multiple lead threads. In this case, the total thread engagement approximates the sum of the circumferential extent of each of the multiple leads. In addition, the multiple leads are circumferentially distributed around the lower portion of the closure skirt to thereby balance the distortional forces involved in closure torquing. On the other hand, multiple lead threads normally require an increased helical angle (vs. horizontal) for the thread and/or an uniformly finer thread. An increased helical angle can lead to closure back-off or unintentional unthreading or even loosening of the thread. In addition, an uniformly finer thread will decrease the amount of radial thread overlap thereby reducing the ability of the system to withstand closure distortions. Such threads will also promote cross threading during application due to the decrease target presented to the closure thread lead by the reduced container thread pitch.
It is clear to those skilled in the art that substitution of LDPE materials for more rigid materials, while accomplishing benefits unique to LDPE, also involves performance tradeoffs which cannot always be recovered by the alternate designs advanced to date.
Additional problems have arisen recently when attempts have been made to employ certain closure designs using certain capping practice. These problems can be broadly categorized as associated with the capping process as opposed to the material choices for the package components.
A first method of capping, known in the industry, involves a “pick and place” operation. This method includes positive positioning of a closure within a gripping chuck which is then moved directly over a container. The chuck is simultaneously turned and moved axially toward the container to screw the closure onto the container finish. This application method is similar to actual manual application. Further details of this application method appear in the “Detailed Description Of Preferred Embodiments” which follows in the Specification.
An alternate, less expensive, approach to closure application can be characterized as a “pickoff” operation. During “pickoff” a closure is held in a chute and positioned at an angle relative to the axis of a container finish that passes beneath the closure. The container finish comes into contact with the closure and picks it off the chute. Unfortunately, the “pickoff” approach can lead to certain difficulties associated with structural design and material selection as will be more fully explained herein in association with prior art FIG. 4. These difficulties and the novel solutions are more fully described in the “Detailed Description of Preferred Embodiments” to follow.